Diverse adhesive technologies have been used for skin contact applications. A review article by Pierson (TAPPI Journal, June 1990, pages 101 to 107) describes the broad range of approaches which have been taken to provide adhesive systems which will adhere to skin and have utility in a diverse number of areas.
One major category of such skin contact adhesives are compositions based on physical blends of hydropholic additives into normally pressure sensitive materials. For example, Chen in U.S. Pat. No. 5,339,546, Osburn in U.S. Pat. No. 4,477,325 and Doyle et al. in U.S. Pat. No. 4,551,490, all teach variations of this art. While effective in some applications, such physical blends suffer in that the hydrophilic additives are not chemically bound into the polymeric backbone. As a result, when becoming hydrated by the exudate of body fluids, such as perspiration, phase separation can occur. This leads to the formation of weak boundary layers and loss of adhesives properties. Further embodiments of this same technique of physically blending hydrophilic additives, such as hydrocolloid powders as powdered sodium carboxymethylcellulose (NaCMC), into a polymeric pressure sensitive adhesives as binders have been proposed, including the use of acrylic pressure sensitive materials as the polymeric binder. Further, the addition of other water soluble, but not chemically bound agents, such as low molecular weight polyols, as glycerol, have been proposed to enhance the rates of hydration (as in International Application WO 91/09633). These too suffer from not having the added hydrophilic agent chemically bound into the polymer system.
It has long been known that pressure sensitive adhesives can be produced from a variety of alkyl esters of acrylic acid. Polymers and copolymers based on C.sub.4 to C.sub.10 acrylic esters are inherently pressure sensitive and have a well established use in industry. The effects of monomer type and of the resultant molecular weight on pressure sensitive adhesive properties are well known to those skilled in the art. For instance, in the Handbook of Pressure Sensitive Adhesive Technology, Satas teaches how the chain length of a dependent alkyl group influences such properties as glass transition (T.sub.g) and resistance-to-peel (pages 397 to 402). Numerous other publications can be found to further illustrate this well known technology.
Attempts to enhance the moisture permeation of acrylic pressure sensitive adhesives themselves have involved such techniques as aerating the adhesive as it is applied to a web and dried, creating a micro structure resembling a polymeric foam. Other attempts to enhance the hydrophilicity of acrylic pressure sensitive adhesives have involved the addition of monomers, such as n-vinyl pyrrolidone or acrylamide, during synthesis of the pressure sensitive adhesive, as discussed by Lucast and Taylor (TAPPI Journal, June 1990, pages 159 to 163). However, the use of said monomers pose toxicity concerns during manufacture and are often used in only relatively minor quantities. Furthermore, polymerization must be carried out in solvent media in order to adequately incorporate these monomers, which contribute to hydrophilicity, into the polymer backbone. The resultant polymer must then be cast from a solvent onto a web in order to create useful products. Such solvent casting techniques now pose environmental concerns over volatile organic emissions and have inherent production inefficiencies. Kellen et al. in U.S. Pat. No. 4,737,559 teaches that such solution polymerization techniques are common and well known in the art.
Hydroxy containing acrylic monomers have long been used in the manufacture of medical products. For example, polymerizates of hydroxy ethyl methacrylate (HEMA) are used in the manufacture of contact lenses. The resultant products made from pure HEMA are hard, brittle plastics which require some plasticization in order to yield functional materials. However, because of their hydroxy functionality, said products are inherently moisture permeable and susceptible to moisture pick-up and are thus inherently hydrophilic. Polymerization of HEMA itself is exceedingly sensitive to reactor conditions (as taught by Bursky et al. in U.S. Pat. No. 4,904,749). Because of the high charge transfer of its --OH group, polymerization of HEMA readily results in gel or partially crosslinked polymer networks and when not gelled poly-HEMA must often be stored under cool or refrigerated conditions to prevent further autopolymerization. Nonetheless, Korol in U.S. Pat. No. 4,563,184 teaches the value of polymerized HEMA adhesive systems as being efficacious in the delivery of certain drugs or bioactive components and in wound healing applications.
Attempts to incorporate HEMA or similar hydroxy containing monomers into acrylic pressure sensitives have, as a consequence, been limited to relatively small additions, say &lt;5%, to the monomer make up of an adhesive polymerizate. Even at these relatively low concentrations, difficulties are encountered in attempting to control the kinetics of synthesis. Often an undesirable polymer gel results when attempting to synthesize conventional, inherently pressure sensitive acrylic copolymers based on C.sub.4 to C.sub.8 acrylic esters to which hydroxy containing monomers, such as HEMA, have been added. Such gels cannot be further processed or applied to webs or substrates as adhesive materials. Thus, the use of hydroxy containing monomers, such as HEMA, has been limited in the use of pressure sensitive adhesives for applications in which the hydrophilicity of HEMA like substitutive groups would be desirable, as for skin contact adhesives.
The chemical structures below are illustrative, but not necessarily inclusive, of the differences between conventional acrylic based pressure sensitive adhesives and the novel adhesives based on modifying a prepolymerized acrylic pressure sensitive precursor with hydroxy ethyl methacrylate.